This invention relates to a reversing permanent split capacitor (PSC) motor design, and in particular, to a reversing PSC motor finding application in a drive system for an agitator of an automatic washing machine. While the invention is described with particular detail in respect to such application, those skilled in the art will recognize the wider applicability of the inventive principles disclosed hereinafter.
PSC motors have been used to drive washing machines for a long time. The motors themselves have been known since nearly the birth of induction motors.
Likewise, washing machines are not new. Over the years, many attempts have been made to simplify drive mechanisms employed to drive the agitator and spin wash basket of automatic washers. Many motor types have been employed for this purpose, including both induction motors and direct current motors of various constructions. More recently, brushless permanent magnet motors, and electronically controlled motors having unusual winding configurations in the form of winding stages have been suggested for use in washing machines. See for example, the U.S. Pat. No. 4,390,826 to Erdman et al. Motors having winding stages are expensive to manufacture and difficult to control. That is to say, they require expensive and sophisticated electronic control circuitry for operation. While conventional brushless permanent magnet motors employing conventional windings, as opposed to the stages used in the Erdman patent, for example, have long been suggested for washing machine applications, they too require relatively complicated control circuitry for operation.
The motor and method of the design disclosed hereinafter utilizes a specifically designed reversible split capacitor motor capable of reversing 120 times a minute to provide the agitation motion for a washing machine. In order to achieve this high reversability, numerous design criteria needed to be met. The criteria to my knowledge, could not be met with conventionally constructed PSC motors.
With respect to any PSC motor where reversing is important, rotor inertia must be kept as low as possible so that the rotor does not develop a tendency to "fly wheel" in the initial direction of rotation instead of reversing as required by the application. The motor also must not have third quadrant torque. This means that when running in a negative speed mode, the motor must not develop any negative torque. If negative torque is developed during operation, the motor may not reverse upon reversal of the power connections. Most importantly, the motor must provide equivalent electrical and mechanical output in both directions. The washing machine performance characteristic necessarily depends on essentially equivalent motor output in each direction of rotation to enable the washing machine to deliver equal washing motions in each direction of motor rotation.
As can be appreciated, the most relevant factor in determining the rotor's inertia is rotor weight. Rotor weight is directly related to its overall diameter. However, if rotor diameter is too small, then the motor is incapable of developing sufficient torque in various drive applications in general and in a washing matching application in particular. On the other hand, if a large bore or diameter design is employed, fourth quadrant torque to overcome inertia is difficult to attain. Under conventional motor design criteria, the motor secondary resistance, that is to say, the resistance of the rotor, is chosen so that it is less than or equal to total stator impedance. The design of the motor described hereinafter employs a high resistance rotor. The secondary resistance, i.e. the resistance of the rotor in the preferred embodiment is approximately 1.4 times the impedance of the stator. In any case, acceptable resistances are in a range between 1.25 to 1.55 times the impedance of the stator. The motor construction described hereinafter achieves certain design criteria not met with conventional techniques. First, it enables the locked rotor torque in each direction of motor rotation to remain high, i.e. approximately 60% of breakdown torque. It also insures that no third quadrant torque will be produced. I have found that when reversible PSC motors designs deviate from these ratios, the result is a motor design that either costs too much, or fails to meet the instant reversability or peak torque requirements of the particular application.
The conventional method for manufacturing a four pole reversing PSC motor is to employ two windings having equal turn counts and wire sizes for the motor stator. The windings then are placed at 90 electrical degrees (45 mechanical degrees) apart in a suitable stator core lamination design. A capacitor is placed between the two windings, and power is applied to one side or the other of the winding/capacitor configuration. Functional use of each winding depends upon which side of the capacitor has power applied to it. With one connection method, the first winding acts as the motor main winding and the second winding acts an auxiliary winding. On reversal, the winding functional aspects also are reversed. Equal winding turns means that the turns ratio or K ratio of the windings, conventionally defined as the number of auxiliary winding effective turns divided by the number of main winding effective turns, is one.
An alternative to the K ratio of one design is disclosed hereinafter, denominated as an open delta connection. In the open delta design, a stator is wound with three windings displaced by 60 electrical degrees. Typically all three windings have the same turn counts and wire sizes. In one direction, one of the windings acts as the main while the combination of the other two windings and capacitor serve as the auxiliary. In the reverse direction the third winding acts as the main while the remaining two windings and capacitor serve as the auxiliary winding. The resulting K ratio in this design is 1.732. Because the K ratio is larger than one, a smaller capacitor can be used in the open delta design than in the equal turns ratio arrangement. Total motor cost may be lower because of the ability to use the smaller capacitor.
In general, it is more cost effective to design stator laminations having silhouettes either in square or in other parallelogram shapes. From a manufacturing standpoint, these shapes can be manufactured with less scrap in the lamination manufacturing operation. I have found that with a square lamination design, it is important that the lamination be designed and the winding placement chosen so that each winding controls approximately identical amounts of amination material, so that electrical performance is equal in each direction of motor operation. When designed according to the principles disclosed herein, a low cost, highly efficient, small size and easy to manufacture motor particularly suitable to act in the drive system of a washing machine is the result.
One of the objects of this invention is to provide a low cost reversing PSC motor design.
Another object of this invention is to provide a low cost PSC motor design having applicability in a drive system for a washing machine.
Another object of this invention is to provide a reversible PSC motor having a high resistance rotor design so that no third quadrant torque develops during normal motor operation.
Still another object of this invention is to provide a reversing PSC motor which includes at least a square or other generally parallelogram shape for the lamination silhouette.
Yet another object of this invention is to provide a lamination design having winding receiving slots formed in the laminations, the number and arrangement of which, in combination with the motor winding placement, is adapted to provide equal maximum flux densities in each direction of motor rotation.
Still another object of this invention is to provide a reversing PSC motor that obtains higher rates of reversal and superior motor performance in applicational use than reversing PSC motors heretofor known.
Other objects of this invention will be apparent in view of the following description and accompanying drawings.